1. Field of the Invention
The present invention relates generally to communications apparatus and more particularly to a radio frequency transceiver having an improved voice actuated carrier and echo suppression circuit particularly suited for use in earth station-to-satellite communication systems and the like.
2. Description of the Prior Art
Among the myriad problems encountered in designing communication systems such as are used in linking earth stations via satellites are those relating to transmitter power drain and echos generated in the transmission loop. In dealing with the first of these problems, transmitters have been developed having voice actuated carrier controls wherein the transmission of a carrier signal is inhibited in the absence of a voice or data input to the transmitter. This obviously reduces the transmission power required from the transmitter during the communication sequence. Such apparatus typically utilizes a fast operating detector for detecting an input signal and a fast operating switching means responsive to the detector for instantaneously actuating the transmitter. However, accompanying such solution is the disadvantage that there is always some first syllable clipping and there is typically some signal degeneration during the transition period required for the transmitter to settle down after having been actuated.
Echo in a telecommunication system typically originates at the far end of a communications circuit where there is frequently an impedance mismatch that results in a reflection of received energy back into the transmit path. As a result, while talking, a talker hears his own speech with a delay proportionate to the transmission circuit link. Such reflection can normally be made tolerable by providing fixed circuit loss in the return path appropriate to the amount of round trip delay. However, when the delay is greater than about 40 milliseconds, the impairment due to the echo becomes intolerable.
Echo suppression circuits having long time delays, such as those used in satellite communication systems, are normally located at both ends of the communications link with each operating independently to react to local transmit and receive levels. Such circuits usually include in the transmit path a unity gain amplifier and a suppression switch which, when activated (during receive mode operation), provides more than 50dB of loss. The control circuitry for the transmit function includes a break-in sensitivity adjustment, a transmit detector and a comparator. In the receive path, an amplifier is provided having the capability of being automatically switched from unity gain to about 6dB of loss. The receive circuit, like the transmit circuit, has a sensitivity adjustment and also includes a receive detector and hysteresis control.
Pursuant to CCITT standards, the suppression circuitry is activated (a loss is inserted in the transmit path) when the receive level is above -31dBmO and is greater than the transmit level. The suppresion switch is inhibited (no loss is inserted) when one of two conditions is met; either the receive level is less than -31dBmO or the transmit level is greater than the receive level. When the transmit level is greater than the receive level the receive path is also conditioned for 6dB loss. The comparator is normally also equipped with a break-in hangover feature which reduces transmit speech clipping. This hangover delay maintains the bi-directional condition during transmit level pauses of less than 200 to 300 milliseconds to prevent loss of syllables due to unnecessary switching. Loss of speech at the beginning of a train of conversation (speech clipping) is usually further reduced by the use of a short time constant in the detection circuitry.
Among the disadvantages of the prior art devices are that almost all suffer from at least some first syllable clipping of the transmission and almost all experience signal degeneration due to the turning on of the carrier during the transmission of the first syllable and due to the use of duplicated components in the separate echo suppression and voice actuated carrier circuits.